1. Field of the Invention
The invention relates to a vehicle control device and control method, and more specifically to break cooperative control by regenerative braking force and hydraulic braking force.
2. Description of Related Art
A motor vehicle having a vehicle driving electric motor mounted thereon, such as a hybrid vehicle or an electric vehicle, performs braking force control when braked, to ensure desired braking force for the overall vehicle by cooperation between regenerative braking force provided by a vehicle driving electric motor and braking force provided by a hydraulic braking device (hereafter, also referred to as hydraulic braking force) (see, for example, Japanese Patent Application Publication No. 2004-196064 (JP-A-2004-196064), Japanese Patent Application Publication No. 2004-155403 (JP-A-2004-155403), and Japanese Patent Application Publication No. 2010-141997 (JP-A-2010-141997)). In the description below, this braking force control shall be also referred to as “cooperative brake control”. Electric power generated by regenerative braking is recovered to charge an on-vehicle electric storage device, whereby the energy efficiency, that is, the fuel economy of the vehicle is improved.
JP-A-2004-196064 describes cooperative brake control of regenerative braking and friction braking in order to prevent occurrence of insufficient vehicle deceleration due to delayed response to friction braking command value when braking is switched from regenerative braking to friction braking. More specifically, it describes a technique in which when the sharing ratio is changed by reducing the sharing ratio of regenerative braking torque while increasing the sharing ratio of friction braking torque, the rate of reduction of the regenerative braking torque is suppressed according to a delay in response of the friction braking torque.
JP-A-2004-155403 describes a cooperative control device of a composite brake which calculates a braking torque due to a hydraulic control error based on a wheel cylinder hydraulic reference model and corrects the command value of regenerative braking torque in consideration of this control error.
JP-A-2010-141997 describes cooperative brake control for a motor vehicle in which a reference value of the upper limit of charging power of an electric storage device, which limits regenerative braking force, is set variably according to a change rate of the upper limit of charging power. This makes it possible to ensure a period of time required for rise of hydraulic pressure during transition from a state in which both regenerative braking and hydraulic braking are used to a state in which only hydraulic control is used.
On the other hand, the application of lithium ion secondary batteries as on-vehicle electric storage devices has been increased. The lithium ion secondary batteries have high energy density and high output voltage, and thus can be used as on-vehicle electric storage devices requiring large battery capacity and high voltage.
However, a lithium ion secondary battery has a problem that lithium metal may deposit on a surface of the negative electrode depending on a status of use, resulting in heat generation in the battery or deterioration in performance thereof. In order to solve this problem, WO 2010/005079 discloses a control technique in which deposition of lithium metal on the negative electrode of a lithium ion secondary battery is suppressed by adjusting the power that is allowed to input to the battery based on its charge and discharge history. More specifically, it describes a technique in which, based on the history of battery current, a maximum current value that will not cause deposition of the lithium metal is successively calculated, while the power that is allowed to input to the battery is adjusted so as not to exceed the maximum current value.
In a vehicle performing cooperative brake control as disclosed in JP-A-2004-196064, JP-A-2004-155403, and JP-A-2010-141997, the fuel economy is improved further as the ratio of regenerative braking force is increased during deceleration. Therefore, in a vehicle using a lithium ion secondary battery as the on-vehicle electric storage device, the sharing ratio of regenerative braking must be increased as much as possible while suppressing deposition of lithium metal.
However, as described in WO 2010/005079, the charging power to the battery must be restricted and thus the regenerative braking force must also be restricted when the charging state becomes such that the risk of deposition of lithium metal is expected. Once charging limitation like this is started, the cooperative brake control must be performed to reduce the regenerative braking force in order to suppress the deposition of lithium metal, while substituting the shortfall caused thereby with hydraulic braking force.
If the regenerative braking force is changed at a high rate, as described in JP-A-2004-196064, JP-A-2004-155403, and JP-A-2010-141997, instantaneous fluctuations may occur in the vehicle braking force due to delayed response of the hydraulic braking force. Such fluctuations in the braking force may possibly give uncomfortable feeling to a passenger in the vehicle even if the braking performance of the vehicles is not affected thereby.